A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction), while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The imaging of the pattern may be performed with a projection system that often includes a plurality of optical elements, such as mirrors and/or lenses. The term “projection system” should therefore be interpreted broadly as encompassing various types of projection systems, including refractive optics, reflective optics, and catadioptric systems, for example.
In a lithographic apparatus, the size of features that can be imaged onto the substrate is limited by the wavelength of the projection radiation. To produce integrated circuits with a higher density of devices, and hence higher operating speeds, it is desirable to image smaller features. While most current lithographic projection apparatus employ ultraviolet light generated by mercury lamps or excimer lasers, it has been proposed to use shorter wavelength radiation, in the range of 5 to 20 nm, in particular around 13 nm.
Such radiation is termed extreme ultra violet (EUV) or soft X-ray and possible sources include, for example, laser produced plasma sources, discharge plasma sources, or synchrotron radiation from electron storage rings. These types of radiation require that the beam path in the apparatus be evacuated to avoid beam scatter and absorption. Because there is no known material suitable for making a refractive optical element for EUV radiation, EUV lithographic apparatus use mirrors in the radiation (illumination) and projection systems. Even multilayer mirrors for EUV radiation have relatively low reflectivities and are highly susceptible to contamination, which further reduces their reflectivity and, hence, the throughput of the apparatus. This may impose further specifications on the vacuum level to be maintained, and may necessitate especially that hydrocarbon partial pressures be kept very low.
Plasma sources produce, as a by-product of the generation of radiation, debris particles that include ions, atoms, molecules, and tin droplets. The ions often have a high speed. Also the atoms, molecules and tin droplets may be ionized due to, for example, photo-ionization. These particles also may have a high speed.
A problem associated with optical elements exposed to the fast particles is oxidation of the top layer of these optical elements. In an attempt to solve this problem, European Patent Application Publication No. EP-A-1 369 744 discloses a capping layer that includes an alloy of Mo and Cr for protection against a chemical attack.
European Patent Application Publication No. EP-A-1 065 568 discloses a capping layer formed of a relatively inert material, for example, diamond-like carbon, boron nitride, or another material resistant to oxidation.
European Patent Application Publication No. EP-A-1 416 329 discloses a capping layer that includes one or more fullerenes. The fullerenes may be provided on an outer layer of a mirror. Being chemically inert, the fullerenes substantially lower the probability of incoming particles sticking to the mirror.
Another problem encountered in multi-layered mirrors is the intermixing of the multi-layers. According to EP-A-1 416 329, fullerenes may be provided between layers of a multi-layered mirror, thereby preventing the layers from intermixing. EP-A-1 416 329 also mentions the use of ruthenium-molybdenum layers as a protective capping layer.
Yet another problem encountered is the sputtering of the outer layer of optical elements due to the impact of the fast particles. This is a damaging effect to the optical elements of the projection system. The mirror surface roughens due to the sputtering, thereby leading to loss of reflection and imaging deterioration. Furthermore, as the removal of material from the mirror surface is a net translation of that carefully positioned surface, the apparatus, and in particular the illumination and/or projection system, may not perform as intended.
European Patent Application Publication No. EP-A-1 186 957 discloses a lithographic projection apparatus having gas supply means for supplying a gaseous hydrocarbon to a space containing a mirror. This hydrocarbon physically or chemically adsorbs to a surface of the mirror and thus forms a protective layer on the surface. When the fast particles produced by the plasma source hit the surface of the mirror, hydrocarbon molecules are dislodged from the protective layer. When this protective layer is too thick, reflectivity of the mirror becomes unacceptably low. Therefore, the thickness of this protective layer should be controlled to avoid a decrease in reflectivity of the mirror, and to maintain the protectiveness of the layer against incoming particles.